The present invention relates generally to optical networks, and in particular to a method for determining optimal location and value of dispersion compensating modules (DCMs) in optical networks.
Dynamic networks have grown in size over the past decades from local area networks (LANS) to metropolitan area networks (MANs) to wide area networks (WANs), as user demand for connectivity has increased. New design issues have arisen and continue to arise as these networks become larger and more complex, necessitating the use of components such as dispersion compensating modules (DCMs) and optical amplifiers.
Determining the location and value of DCMs in MANs is a design issue that has arisen since the growth in the size of MANs has reached the degree that dispersion compensation has become necessary in MANs. Minimizing the total number and values of DCMs in the network is the motivating factor in DCM placement methods for MANs because smaller, dynamic networks, such as MANs, are cost-sensitive.
Currently, there are existing methods for determining the location and value of DCMs in a MAN whereby DCMs are placed on a selected number of fiber spans in the network. These methods are performed manually using the intuition and experience of a designer to select the locations and values of DCMs in the networks, as is illustrated by the following document.
An optical university project by L. Chrotowski, C. Mateus, F. Mo, and L. Zhou at the University of California, Berkeley dated Dec. 17, 2001 and entitled xe2x80x9cOptical Network Design of a Metro Ringxe2x80x9d discloses a method for DCM design in a metro ring network involving quantifying the degree of eye closure on a signal, which is used as the factor upon which the DCM placement is dependent. The placement itself however is determined heuristically by the designers, who attempt to minimize the total number of DCMs in the network by determining placement of just enough DCMs so that the network is operating within desired conditions (in this case to a maximum value of eye closure).
However, real-world MANs are topologically complex and often take the form of rings or meshes that may include coupled lightpaths. With increasing size and complexity of MANs, the manual, heuristic methods of DCM placement become impractical and inefficient.
A systematic method for determining location and value of DCMs allows efficient placement of DCMs in a variety of network topologies, as illustrated in the following patent application. U.S. patent application Ser. No. 5,559,920 to Ng et al. filed Sep. 24, 1996 and entitled xe2x80x9cMethod and system for determining location and value of dispersion compensating modules in an optical networkxe2x80x9d discloses a DCM placement procedure that comprises evaluating possible DCM values and locations and successively adding selected combinations to the network until the dispersion limits of the network are met. The method, however, does not guarantee optimality, optimality being the ability to maximize or minimize a given variable such as total dispersion, number, or cost of DCMs in the network.
Therefore, there is a need in the industry for the development of a systematic method for determining the location and value of DCMs in an optical network that would provide an optimal solution.
Therefore there is an object of the invention to provide a method for determining the optimal location and value of DCMs in an optical network that would avoid or minimize the above-mentioned drawbacks.
According to one aspect of the invention, there is provided a method for determining the optimal location and value of one or more DCMs in an optical network, comprising the steps of:
(a) determining a lightpath topology in the network;
(b) introducing and initializing a data structure having multiple entries, each entry in the data structure being used for storing DCM locations and values in the network and a score measuring the effectiveness of dispersion compensation in the network by the stored DCMs;
(c) extracting the entry from the data structure, which has the lowest score and determining if the effective dispersion on the lightpaths in the network having the stored DCMs from the extracted entries are substantially zero, the effective dispersion being an amount of dispersion accumulated along a lightpath that exceeds the maximum positive dispersion value Pos_Disp_Limit specified for the network;
(d) if the effective dispersions on a lightpath is not substantially zero, expanding the extracted entry into multiple entries by adding available combinations of DCM location and value to the extracted entry;
(e) calculating a score for each expanded entry and discarding those entries that cause the accumulated dispersion on any lightpath to be less than the maximum negative dispersion limit Neg_Disp_Limit of the network;
(f) inserting the expanded entries into said data structure; and
(g) repeating the steps (c) to (f) until the effective dispersions are substantially zero for the extracted entry in the step (c).
Advantageously, the step (d) of expanding comprises expanding the extracting entry into multiple entries by adding every available combination of DCM location and value to the extracted entry.
The step of introducing and initializing the data structure comprises introducing and initializing the data structure which is a priority queue, including maintaining the entries in an ascending order according to the score; and the step (c) of extracting the entry with the lowest score comprises extracting the first entry from the priority queue.
A method may further comprise the step of maintaining the priority queue in an ascending order according to the score, comprising sorting the priority queue, the step being performed after the step (f).
Conveniently, the step (f) may comprise inserting the expanded entries into the priority queue so that the priority queue maintains the ascending order according to the score.
Beneficially, the step of determining the lightpath topology comprises:
identifying lightpaths in the network;
assigning lightpath identification numbers to the lightpaths; and
identifying fiber spans over which the lightpaths are laid.
The step of identifying lightpaths in the network may comprise identifying all lightpaths in the network including protection lightpaths and reconfigurable lightpaths.
The step (e) of calculating the score conveniently comprises calculating the score to be equal to the sum of:
the total value of the DCMs stored in the entry; and
the remaining effective dispersion in the network divided by the number of lightpaths having remaining effective dispersion.
For example, the step (e) of calculating the score may comprise calculating the score to be equal to the sum of:
xcexa3wixc2x7DCMmin, wherein DCMmin is the smallest DCM value to be used in the network, and wi is the weight factor for the corresponding DCMi stored in each expanded entry; and             g      ⁡              (        x        )              ·          min      ⁡              (                                            w              i                        ·                          DCM              min                                            DCM            i                          )              ,
wherein g(x) is the remaining effective dispersion in the network divided by the number of lightpaths having remaining effective dispersion, and   min  ⁢      xe2x80x83    ⁢      (                            w          i                ·                  DCM                                    xe2x80x83                        ⁢            min                                      DCM        i              )  
is the smallest value of       (                            w          i                ·                  DCM                                    xe2x80x83                        ⁢            min                                      DCM        i              )    .
Advantageously, the step (c) of determining if the effective dispersions are substantially zero comprises measuring the effective dispersions in units of distance.
If required, a method may further comprise the step of determining alternative DCM locations such that the transfer of the DCM to the alternative location does not change the accumulated dispersion along any lightpath in the network.
In a method described above, the step of introducing and initializing the data structure may comprise introducing and initializing the data structure, which is a series of priority queues, the series being maintained in an ascending order according to the score of the first entry of the priority queues, and the step (c) of extracting the entry with the lowest score may comprise extracting the first entry from the first priority queue in the series of priority queues.
Conveniently, the method provides the optimal location and value of one or more dispersion compensating modules (DCMs) in an optical network, wherein the optimal location is being defined as providing one or more of the following:
an optimal total dispersion in the network;
a minimal number of DCMs in the network; and
a minimal cost of DCMs in the network.
The methods for determining the optimal location and value of DCMs in an optical network of the embodiments of the invention provide a systematic procedure that is efficient and applicable to a variety of network topologies.